The Archbishop of Canterbury has told MPs to oppose an amendment to the gay
marriage bill which would allow heterosexual couples to have civil
partnerships.

In a briefing to MPs, the Archbishop warns that the amendment does not have any "clear social good" and will only create "further confusion" about the role of marriage in society.

As many as 150 Tory MPs are expected to join forces with Labour and Liberal Democrat MPs to vote for the amendment, which the Government has said could derail the whole same-sex marriage bill.

Maria Miller, the Culture Secretary, has warned that giving heterosexual couples the right to enter civil partnerships would "throw up significant challenges" and mean a "significant delay" to legislation.

The Church of England's briefing, which has been published today, says: "We believe that this would introduce further confusion about the place of marriage in society.

"We remain unconvinced that the introduction of such an option would satisfy a genuine and widespread public need, other than for those who pursue 'equality' as an abstract concept.

"There has been little public evidence to suggest that significant numbers of opposite-sex couples who choose not to marry would opt instead for a civil partnership.

"We are not convinced that any clear new social good is created by this further innovation in civil partnerships and therefore they are best left as they are at a time when considerable uncertainty is being caused by the fundamental change in the nature of marriage."

However, The Church of England has also voiced strong opposition to the gay marriage bill, which it warns will have "uncertain and unforseen consequences for wider society".

It says that Church of England schools should be free to teach traditional views of marriage, and that Christian registrars should have the right to opt out of conducting same sex marriages.

"We do not believe it would be acceptable for marriage registrars who are in post prior to the Bill's enactment to face disciplinary actions or redundancy simply as a result of their conscientious views on marriage," the Church says.

The Government has suggested that extending civil partnerships would hit the Treasury with an extra £4 billion in pension liabilities.

Senior Tories have urged their colleagues to support the law when the Marriage (Same Sex Couples) Bill returns to the Commons today for the report stage and third reading.

Speaking to BBC Radio 4’s Today programme, Mrs Miller hinted that the entire future of the Bill could be threatened because of the amendment.

She said it would mean lengthy consultations and complicated policy changes.

“I think it would make a fundamental difference to the passage of the Bill because what the amendment is trying to do is to import in a very complicated concept which took more than 200 clauses to explain in the original Civil Partnership Bill into a Bill which is all about extending marriage and it’s trying to do that in just two very short amendments,” Mrs Miller said.

“It introduces complexity which would not only, I believe, delay the passage of the bill through Parliament, but it would also put forward a number of quite fundamental policy issues which need considerable thought and consultation, let alone any delays around implementation."

Mrs Miller added: “I think it would throw up significant challenges - obviously it would be for us to go through a consultation and a consideration period to be able to understand how that would work because at the moment there really isn’t the detail there.”

More than 30 local Conservative chairmen signed a letter to the Prime Minister over the weekend accusing Mr Cameron of “utter contempt for ordinary people” over the issue of gay marriage.

Bob Woollard, chairman of the Conservative Grassroots group which organised the protest letter to the Prime Minister said: "Same sex marriage is really a tipping point, a bellwether issue if you like - people have just said 'I've had enough, I'm off, I will never vote Conservative again'.

Nick Herbert, the former Home Office minister, has admitted that grassroots Tories could leave the party because of the row.

“Absolutely none of us either want to upset or lose activists,” he told BBC News.

However, Mr Herbert added that the Bill does “no harm at all” because churches will not be forced to hold same-sex services.

Helen Grant, the Equalities Minister, said that “there is no demand” for civil partnerships from heterosexual couples.

An Ipsos Mori poll in December found that 73 per cent of British adults thought that gay people should be allowed to get married.

It comes as Tory donor Lord Ashcroft warned that rows over an EU referendum and comments made by a senior Conservative about “swivel-eyed loons” threaten to leave the party in a “spiral of irrelevance”.

Writing on the ConservativeHome website Lord Ashcroft said: “The Conservatives were elected, albeit in insufficient numbers, to do a job.

“We hope to be elected in two years' time in rather greater numbers to implement a Tory manifesto in full. But we don't look as if either of those things is our priority.”